Hi again all
I know it sounds extreme ... but if the motherboard is still under warranty, it might be worthwhile to try a different board. I'd mentioned another ASUS Z170 user who had success by switching to an MSI Z170 board. Many of the same features, little of the same headaches (everything worked OK from the get-go, as a matter of fact). If you want to view that thread (it's rather remarkable how many system components are the same in your system and his)... it's here in the Windows 10 forum, the original poster is BestLux.
I don't think that every ASUS Z170 board has these stutter issues, but it's looking more and more like some certainly do. It might be nothing more than a production problem, and some boards have a defect. Might be hard for quality control to detect (but not so hard for the unlucky few who get the problem boards).
Hard to know! ... One thing is certain: systems with high-end components should produce high-end performance. You've paid good money, and the vendors need to come through.
One way or another -- hope it works out. You've been throwing a lot of effort into the troubleshooting [xilolee too -- you guys are marathoners!] ... I hope a resolution shows up soon!